Human health requires the ability to withstand colonization and infiltration by microorganisms from diverse environmental sources. Many well-known infectious diseases--such as AIDS, malaria, and tuberculosis--are caused by microorganisms that are professional human pathogens. These organisms occupy a place near the center of biomedical research awareness. Surrounding this center is a diffuse galaxy of environmental organisms that inflict significant morbidity and mortality in humans, but are not professional human pathogens. These agents are responsible for devastating forms of disease that can affect large populations, subpopulations of susceptible individuals, have serious economic impact and/or serve as deadly biowarfare agents. Environmental pathogens, defined as human pathogens that are acquired from environmental reservoirs, pose unique challenges. A common feature of environmental pathogens is that their traits, including the ability to infect susceptible humans, have evolved and are maintained in non-human environments. This raises intriguing fundamental questions. How do environmental organisms gain the ability to infect humans? Does the answer tell us anything about the evolution of infectious diseases on earth? What adaptive and regulatory mechanisms do they use to make the transition between nonhuman and human environments? Are environmental species heterogeneous with regard to infectivity, and, if so, how to we assess the risk of infection? Emerging challenges, such as global climate change and the threat of biowarfare, could radically alter the nature of human interactions with environmental pathogens, affecting disease prevalence in ways that are difficult to predict. Even without such challenges, the prevalence of diseases caused by environmental pathogens is, for the most part, unknown. This poses a challenge for environmental regulators, drinking water utilities, and others who need to know which environmental pathogens they should worry about. The science of environmental pathogens often falls into a gray zone between infectious disease research and environmental microbiology. The American Academy of Microbiology plans to convene a group of international experts for a colloquium February 6-8, 2004, in Tucson, Arizona, to address the problems and challenges associated with environmental pathogens. A group of approximately 30 scientists will be invited to participate in this colloquium. We plan to bridge the gaps in communication that exist between public health professionals, medical researchers, environmental health scientists, and those concerned with homeland security. Following the colloquium, a report will be developed--in both print and electronic formats--that will be analytical and comprehensive, yet offer practical recommendations for the future.